The present invention relates to a hydrostatic two-way-variable-speed drive, particularly for motor vehicles.
Hydrostatic drives are known substantially comprising a hydraulic pump powered by a drive shaft, e.g. of a vehicle internal combustion engine; and a hydraulic motor connected mechanically to the vehicle wheel drive, and hydraulically to the aforementioned pump by which it is supplied with and powered by pressurized oil.
Drives of the aforementioned type are provided with means for continuously varying displacement of the pump and motor, for supplying the motor with predetermined quantities of pressurized fluid. Means are also provided between the pump and motor, for alternately connecting the pump cylinders with those of the motor.
Drives of the type briefly described above present a number of drawbacks.
Firstly, the speed range of the shaft controlled by the hydraulic motor is fairly narrow. On known drives of the aforementioned type, in fact, the gear ratio is normally varied by varying displacement of the pump or motor, so that maximum speed corresponds with maximum displacement of the pump or minimum displacement of the motor.
Secondly, overall efficiency of the drive is fairly poor, due to hydraulic pressure losses, particularly under certain operating conditions of the drive.
Thirdly, drives involving the connection, as described above, of a hydraulic pump and motor are bulky, particularly in the direction of the pump axes, thus making them unsuitable for use on vehicles with a fairly small engine-transmission compartment.
Finally, drives of the aforementioned type are heavy and extremely noisy.